add dnsname as separate label, use domain label consistently

The domain coming from the config is already used to indicate fetch success, which is confusing in cases where dnsname (what's in the certificate) might differ from what's in the config.

This is a breaking change, since the content of the domain label is now different.

add timeout

fix crash when request fails

add config for self signed tls, improve error handling

[Fix #1] close connection once metrics were pulled

update readme, add service file